All known projects currently at the design or implementation stage have their technical basis in the principle put forward by the physicist Arsene d'Arsonval and in turn based on Carnot's principle. This concerns the operation of a thermodynamic machine using water taken from the surface of the sea as the "hot" source and water from the depths of the ocean, which is continuously protected from the action of the sun, as the "cold" source.
The principal known floating devices designed for conversion of the thermal energy of the sea into usable kinetic energy fall into a number of groups, depending on the type of floating support structure used:
boats or barges with a substantially rectangular flotation area,
circular floats in the general shape of a biscuit, whose flotation area is a large circle,
spherical floats,
submersible cylindrical floats,
vertical column type floats, and
semi-submersible structures.
Experience has shown that the semi-submersible structure type of support offers numerous advantages, in particular in that it is more resistant to the action of the ocean swell than a floating structure having a large flotation area.
Two interesting projects already propose the use of this type of structure.
A first proposal is a semi-submersible offshore station of the Froude pole type. A cylindrical concrete structure carries a downwardly extending tube, also of concrete, for taking up cold water. This structure supports four identical modules arranged around its perimeter, each module comprising an evaporator and a condenser separated by an axial-flow turbine-driven generator set. The modules are removable and may be replaced in their entirety for repair and maintenance. Although of great interest, this project has a number of disadvantages:
it features no self-contained means for manipulating and replacing the modules,
the modules must be connected up under water, calling for the use of divers,
the procedures for avoiding pollution of the gas circuits by sea water during replacement of modules are complex,
the fluid circuits are complex,
the turbine-driven generator set uses a submerged motor, of doubtful reliability and preventing in-operation maintenance,
the heat exchangers (condenser and evaporator) are of a highly sophisticated design,
the modules are vulnerable in that they are disposed around the perimeter, and therefore susceptible to damage by impact with floating bodies,
the stability of the platform is compromised in the event of entry of water into any of the peripheral compartments,
equilibrium of the structure is lost when any module is removed.
This structure uses a closed circuit for its cryogenic fluid, which is ammonia. Water is circulated through the inside of the heat exchanger tubes and the ammonia over the outside.
The second proposal is a platform whose support structure is of concrete, but in this instance the platform is of circular shape, comprising four groups of evaporators, turbine-driven generators and condensers. These groups are not designed as interchangeable modules as in the previously described project, but are integrated into the shell of the structure in four respective machine rooms. This platform also uses a closed ammonia circuit.
Apart from the fact that this type of platform is subject to high levels of movement under the effect of the ocean swell, this project has further disadvantages:
the condensers and evaporators are of a highly sophisticated design,
the cryogenic gas and water circulation circuits are numerous and complex,
the dismantling and replacement of component parts of the integrated groups is complicated by virtue of the large number of circuits and their connections to the components in question,
stability may be lost in the event of damage to any compartment, leading to the entry of water.
Other known projects differ in terms of the type of floating support structure and the arrangement of the component parts of the power groups. They also suffer from the aforementioned disadvantages, to a greater or lesser extent.
The present invention is intended to provide a platform offering a higher level of performance, reducing or eliminating the stated disadvantages of the prior art platforms while being of simple design and offering moderate manufacturing and maintenance costs.